Expression of the immediate early genes of herpes simplex virus I requires the assembly of a multiprotein RNAPII enhancer complex consisting of several viral and cellular transcription factors. This assembly provides a model for the analysis of the components involved in the specific activation of gene expression. The present study focuses upon the characterization of the mammalian C1 factor; a protein complex which interacts with the viral transactivator and is required for the enhancer assembly. Analysis of isolated cDNAs encoding the C1 factor indicate that it is a novel family of proteins which are specifically proteolytically processed from a large precursor. These proteins are generated by site-specific cleavage within a reiterated amino acid sequence. As the processing and assembly of this factor influences its activity, genetic screens have been designed for the analysis of the cleavage sequences and the isolation of the specific protease from mammalian cells. These studies have resulted in the identification of a number of novel factors which interact with the reiterated sequences of the C1 factor. The development of specific antiseras have determined that although the C1 factor is ubiquitously expressed in most tissues, the protein is localized to specific subnuclear structures in certain cell types. In addition, C1 factor homologs and have been identified and isolated from other animal systems to allow for the genetic analysis of these factors. The continuing studies focus upon questions concerning the role of the C1 factor processing in the transcription of the herpes simplex virus genes, the specific transport/localization of the protein, and the development of animal systems for the analysis of the functions of the protein in viral and cellular functions.